This invention relates to an electronic controlled fluid suspension system for altering the steering characteristic of a vehicle.
In order to enhance ride comfort, maneuverability and steering stability of the vehicle, various suspension systems have been proposed which prevent the vehicle from pitching and rolling by supplying and discharging fluid to and from the fluid actuators thereof. For example, a system, disclosed in Japan published unexamined utility model application No. 60-174609, prevents a vehicle from rolling by supplying and discharging fluid to and from fluid actuators. The valve energizing time interval for the fluid supply and discharge is obtained based on a map of vehicle speed and steering angle predetermined in response to steering speed, or a map of vehicle speed and steering speed. Another type of system, disclosed in Japan published unexamined patent application No. 61-193909, controls vehicle attitude, namely, prevents the vehicle from rolling and pitching by supplying and discharging fluid to and from fluid actuators. Relative sprung and unsprung displacement is detected, and feedback control is executed so that the actual displacement should conform with target displacement.
However, in this prior art, when fluid is supplied into and discharged from fluid actuators at the same time to control the vehicle attitude, or when fluid is supplied into and discharged from fluid actuators under open loop control according to an expected change in vehicle attitude, actual supplied and discharged fluid differs in volume from expected fluid due to inconsistent quality in control valves, fluid actuators, etc. caused during manufacturing. As a result, an incorrect load is applied to the wheel fluid actuators A1, A2, A3 and A4, so that the front wheels are turned in a direction different from that of the rear wheels, as shown in FIG. 33. The load on the actuators A1 and A4 is heavy, and the internal pressure thereof is high. Accordingly, it becomes hard to supply fluid to the actuators, and the energizing time interval of control valves lengthens. As the valve energizing time interval lengthens, the internal pressure of the fluid actuators becomes more unbalanced and large torsional force is likely exerted on the vehicle. Such conditions sometimes arise even when feedback control over displacement is carried out and the fluid supply to and discharge from the fluid actuators is controlled.
Even when the front wheels are thus turned in a direction opposite to the rear wheels, the rigidity of the vehicle keeps the vehicle level. However, when the torsional force is applied to the vehicle, a single front wheel or a single rear wheel has to support the vehicle weight. Accordingly, a problem is that the running characteristic largely changes, for example, the friction coefficient of wheels decreases, or the steering characteristic of the vehicle changes from over-steering to under-steering.
In order to change steering characteristic by altering the distribution of roll stiffness among the wheels of a vehicle through suspension control when a vehicle corners, various suspension systems have been proposed. As shown in FIG. 26, the cornering power and the load on the inner and outer wheels of a cornering vehicle have a non-linear relationship. When the load moving between the inner and outer wheels is small as shown by an arrow a, the sum of inner-wheel side load CP2I and outer-wheel side load CP2O corresponds to the cornering power On the other hand, when the load moving between inner and outer wheels is large as shown by an arrow b, the cornering power is the sum of innerwheel side load CP1I and outer-wheel side load CP10. The above cornering powers have the following relationship: EQU 2CPX&gt;(CP20+CP2I)&gt;(CP10+CP1I)
In this way, when the vehicle corners, the smaller the load moving between inner and outer wheels, the larger the cornering power is.
The vehicle's steering characteristic is determined by the formula below: EQU Cr.multidot.Lr-Cf.multidot.Lf=Z, where
Cr relates to the cornering power of rear wheels, Lr the distance between rear-wheel axle and vehicle's center of gravity, Cf the cornering power of front wheels, and Lf the distance between front-wheel axle and vehicle's center of gravity. A negative value of Z indicates an over-steering characteristic of the vehicle, a zero value indicates a neutral steering characteristic and positive value indicates an under-steering characteristic. When the vehicle corners, a front-wheel moving load is controlled to be a minimum, and consequently the front-wheel cornering power increases, resulting in an over-steering characteristic of the vehicle. Alternatively, when the front-wheel moving load is controlled to increase, the front-wheel cornering power diminishes, resulting in an under-steering characteristic of the vehicle.
For example, in a system disclosed in Japan published unexamined patent application No. 61-193908, a suspension system is controlled by compensating for the difference between target and actual sprung and unsprung strokes based on a value of the detected lateral acceleration. The steering characteristic of the cornering vehicle can be arbitrarily changed by setting the laterally moving load of front and rear wheels arbitrarily according to lateral acceleration.
Yet, as for the conventional vehicle, a stability factor Kh has the following relation with a yawing resonance frequency fy. The ratio of yaw rate YR to steering angle MA is shown in the formula below: ##EQU1## L is wheel base, Wf and Wr are the grounded load of the front and rear wheels, M is the mass of the vehicle, I is the moment of inertia in a direction of yaw, V is the vehicle speed, and N is the gear ratio of the steering wheel.
A dynamic characteristic of the vehicle's yaw rate establishes the graph in FIG. 25, in which the ordinate plots the ratio of yaw rate YR to steering angle MA and the difference in phase between the steering angle and the yaw rate, and the abscissa plots a frequency, i.e., steering speed.
More specifically, when the inner and outer-wheel suspensions receive roll of the vehicle body as shown by a solid line in FIG. 25, the vehicle yaw rate varies with steering speed. As the steering speed accelerates, a phase lag develops, and responsiveness to yaw is impaired. A problem is that the yaw rate does not respond to steering speed rapidly due to phase lag, and that dull-responsive steering results. When the vehicle corners, an over-steering characteristic is set by reducing the front-wheel moving load. Then, the front-wheel cornering power Cf increases as aforementioned. According to the aforementioned formula, the stability factor Kh decreases, the ratio of yaw rate YR to steering angle MA increases, and the steering effectiveness also increases. However, another problem is that the yawing resonance frequency fy, namely, the responsiveness to yaw diminishes.
This invention further relates to an electronic controlled fluid suspension system for controlling vehicle attitude by supplying and discharging fluid into and from fluid actuators.